+1 for stevenjohn21's suggestion. I was out on Sunday and picked up Mr. Clean Magic Eraser Original and tried them on a tank that had some stubborn GSA/GDA. It worked great, much easier than most algae pads. Then I tried in on my 20+ yo Versa-top. It did well on the typical water spots but I still had to use a single edge razor blade scraper to remove the stubborn calcium deposits.

use hydroloric acid on the versa top, it will look mint condition after that.